The trees growing on urban streets are subject to a number of hazards, and may appear sickly. Disasters such as the epidemic of 'Dutch elm disease' and the death of trees from the use of de-icing salt are noticeable to city residents and have raised awareness of 'urban forestry'. The most important problems for the vitality of city trees listed in the order of significance are: dryness, salt pollution and nutrient deficiency. In smaller cities, mechanical damage by motorized traffic, construction projects and vandalismpredominate. Other factors considered to be responsible for the deteriorating condition and stunted growth of urban trees are natural gas, acid rain, fumes from industry and traffic, over-shadowing by tall buildings, reduction in the air content of soil by excessive compaction of ground and improper planting and lack of subsequent maintenance.
In British villages, towns and cities there are an average of 17.4 trees per acre, mostly in residential neighbourhoods. In 1993, 60% of British urban trees showed signs of sickness such as leaf loss and discoloration, a worse level than that in many of other 33 countries surveyed. Half of the trees planted in cities die within 5 years from lack of water, nutrient-poor soil and lack of regular attention.
Research shows that trees make cities more liveable: they consume carbon dioxide, provide shade and moisture, harbour wildlife, increase neighbourliness and property values, and are aesthetically pleasing.